1. Field of the Invention
The present invention generally is directed to means and methods for controlling the speed and/or torque of motors. More specifically, the invention is directed to means and methods for controlling the speed and/or torque of single phase induction motors.
2. Description of the Related Art
Single phase induction motors (SPIMs) constitute the majority of motors used in home appliances today. SPIMs include permanent split capacitor (PSC) motors, split phase motors, permanent magnet synchronous motors, and shaded pole motors. As is known, these motors are inherently single speed motors and are used in clothes washers, clothes dryers, dishwashers, hermetic compressors, fans, pumps, draft inducers, et cetera.
SPIMs, generally are supplied power from an AC utility line at 50 or 60 Hz and at 120 or 230 volts. These motors typically have a power rating of between about 5 to about 1500 watts.
As mentioned above, these motors operate at constant speed when supplied from the AC utility line. When multiple speeds are needed, techniques like multiple windings with different number of poles, or tapped windings are used. In some applications, like fans, blowers, or pumps, the applied voltage to the motor can be reduced in order to reduce motor speed.
In industrial applications, three phase induction motors have been used instead of SPIMs due to the general availability of three phase power, as well as the higher output capabilities, higher torque production, higher efficiency and lower inrush currents of the three phase motors. Multiple speeds and torques typically are provided utilizing controllers that apply three phase power to the motors in such a fashion as to effect the variable speeds and/or torques. However, in residential appliances, SPIMs are preferred due to the greater availability of single phase power.
To date, the design of controllers for SPIMs has been greatly influenced by the design of industrial motor controllers. These influences range from developments in power electronics, motor design and control theory. In that regard, variable speed motors, notably induction motors, driven by hard switching three phase inverters are commonly used in industrial applications. However, as alluded to above, low cost, high efficiency, small size, and high production volume are not the main driving force behind industrial motor controller designs. Hence, most industrial motor control schemes are not adaptable for or matched to the appliance industry.
Oftentimes, a SPIM control system will convert fixed frequency AC power (e.g., 120 v, 60 Hz) to DC power, and then invert the DC power to suitable AC frequency power.
The following patents, incorporated herein by reference, disclose various means and methods for driving SPIMs: U.S. Pat. Nos. 4,446,706; 5,252,905; 4,467,257; 4,566,289; 5,214,367; 5,241,257; 3,588,650; 4,281,276; 4,243,927; 4,099,108; 4,488,102; 4,767,976; 5,179,336; and 5,276,392. While these patents disclose various schemes for driving SPIMs at various speeds, U.S. Pat. No. 5,252,905 discloses a variable frequency supply for operating a SPIM in which a Pulse Width Modulated (PWM sine wave is applied to the SPIM. The control circuit includes switches to bypass the electronic switching circuit when it is desired to operate the motor from the 60 Hz line supply. Also, U.S. Pat. No. 4,566,289 discloses a refrigerator control system including an inverter for operating the compressor motor at variable speeds, and switches for connecting the compressor motor directly to the line voltage supply when desired. Finally, U.S. Pat. No. 3,588,650 discloses a variable frequency and magnitude supply for operating a SPIM at variable speeds.
Thus far, control systems that convert fixed frequency AC power to DC power and then invert the DC power to a variable amplitude and frequency voltage to operate a single phase induction motor do not optimize the control for the characteristics of the motor over the full range of operating conditions. Very often the control will limit operating conditions such that the motor will not make audible noises or the control circuit components will not overheat. These controls will often limit their operation to signals of line frequency or less.